But Williams herself was unbreakable, and she held her nerve two win two consecutive tiebreakers against Stosur to close out a 7-6(7), 7-6(7) victory in one hour and fifty-four minutes.

“She was serving so well so I told myself, ‘I can’t be broken because if I’m broken she’ll probably end up winning the set,’” said Williams, who will face either Flavia Pennetta or Taylor Townsend in the third round in Cincinnati.

Williams hit 12 aces and saved the only break point she faced to keep the former U.S. Open champion at bay on a day when Stosur’s serve and forehand were clicking on all cylinders.

“I think I played well, considering how amazing Sam played,” said Williams. “I’m trying not to be so hard on myself.”

Williams fell behind 4-0 in the first set tiebreaker, but last year's Cincinnati runner-up would rally and eventually draw even at 4-4 when she picked off an ill-placed Stosur volley and ripped a forehand passing shot for a winner.

After Stosur double-faulted at 5-all, Williams would fail on her first two set points before closing the opener with a service winner on her third.

The second set followed a similar script.

Stosur moved ahead in the tiebreaker and earned two set points at 6-4, but three straight forehand errors gave Williams her first match point. She would double-fault on it but convert on her second match point a moment later when Stosur netted a backhand volley.

Kvitova Bows out to Svitolina

Petra Kvitova has lost two of three matches after winning Wimbledon. The world No. 4 bowed out to Elina Svitolina of the Ukraine in straight sets, 6-2, 7-6(2).

The victory is the 19-year-old Svitolina’s first against a top ten opponent. Svitolina is the second-highest ranked teenager on the WTA Tour behind Madison Keys.